Ground terminals are in widespread use in satellite communications systems today. When ground terminals are installed, they are turned on and acquire satellite signals. When the satellites are geosynchronous satellites, such as television broadcast satellites in common use today, acquiring satellite signals at turn-on is not a difficult problem. Geosynchronous satellites appear stationary relative to a fixed point on earth, and so it is relatively simple to point a ground terminal antenna to acquire a geosynchronous satellite signal.
Non-geosynchronous satellites present a different situation. Because non-geosynchronous satellites move relative to a fixed point on earth, a ground terminal installer cannot simply point a ground terminal antenna to a fixed location in space and expect the ground terminal to acquire a non-geosynchronous satellite signal. Instead, information regarding the satellite's location as a function of time is needed so that the ground terminal can locate the non-geosynchronous satellite, and acquire its signal.
Historically, ground terminals have been pre-programmed at the factory with information describing the satellite's orbit. Unfortunately, this information can be become old, or "stale," because the orbits of non-geosynchronous satellites can regularly change as a result of orbit degradation and adjustment. The result is that, when ground terminals are turned on, they spend time searching the sky for non-geosynchronous satellites. Since it is desirable to increase the time spent providing services and generating revenue, it would be advantageous to minimize time spent by ground terminals searching for non-geosynchronous satellites.
Accordingly, a need exists for a method and apparatus for efficiently acquiring and tracking non-geosynchronous satellites.